1. Field of the Invention
The present invention relates to a light-transmitting optical member, and more particularly, to a light-transmitting optical component (such as a lens, prism, plate, etc.) that is used in an optical system (such as a lithography apparatus, CVD device, laser processing device, etc.) with a light source (such as an ArF excimer laser, F.sub.2 laser, solid state laser, etc.) having a wavelength below 200 nm. The present invention also relates to a manufacturing method and an evaluation method for such a light-transmitting optical component.
2. Discussion of the Related Art
An optical lithography process uses a lens (light-transmitting optical member or component) to direct light from an exposure light source through a mask to a wafer coated with a photo-sensitive material, thereby transferring a pattern on the mask onto the wafer. LSIs are being developed with increasingly high density. Generally, the resolving power of the transferred pattern is inversely proportional to the numerical aperture of the lens and directly proportional to the exposure light wavelength. The numerical aperture of the lens increases as the diameter of the lens increases. However, it is difficult to increase the numerical aperture of the lens past a certain limit, because an impractically large diameter is required. For this reason, a reduction in the wavelength of the light source is desired in order to allow a further improvement in resolution in an optical lithography process.
Until now, light sources used in optical lithography have utilized ultraviolet light including the i-line (365 nm) of a high pressure mercury lamp and a shorter wavelength light (248 nm) of a KrF excimer laser. Because the KrF excimer laser is capable of high power output with a high laser oscillation frequency, the KrF excimer laser has been widely used as an efficient light source for optical lithography, and research and development continues to increase its efficiency. As a result of this wide use of ultraviolet light, optical lenses with high ultraviolet transmissivity have been increasingly used in the optical systems of optical lithography apparatus.
Recently, to improve the resolution further, it is expected that light sources producing vacuum ultraviolet light of even shorter wavelengths will be used in optical lithography apparatus.
Although high-light-transmittance optical lenses are used in conventional optical lithography apparatus, these conventional lenses cannot provide the practical degree of transparency needed with vacuum ultraviolet light of a shorter wavelength (about 200 nm or less). This problem has prevented the use of vacuum ultraviolet light sources in optical lithography processes. Vacuum ultraviolet light has a high energy, more than about 6.2 eV. Therefore, if the transmittance of a lens is not sufficiently high, the energy which is not transmitted by the lens is converted to heat, and as a result, the imaging performance deteriorates due to thermal expansion of the optical lens. Also, with such an insufficient transmittance, a photo-resist cannot be properly exposed.